This invention relates generally to industrial gas turbines used in integrated gasification, combined cycle (IGCC) systems. Specifically, the invention relates to directly injecting nitrogen into the combustor of the gas turbine to reduce total NOx (nitrous oxides emissions) from the IGCC system.
FIG. 1 shows an exemplary IGCC system which includes a gasification system that is integrated with power producing turbine systems. A gasifier 10 converts a mixture of fuel, air or oxygen, steam and optionally limestone into an output of hot fuel gases. These gases are cleaned in a clean-up device 12 and supplied to the combustor 14 of a gas turbine 16. The power output from the gas turbine drives a generator 18 that supplies electrical power to a power grid 20. Hot exhaust from the turbine of the gas turbine is supplied to a heat recovery steam generator 22 which produces steam that drives a steam turbine 24 and flue gases which are exhausted from the IGCC system. Power generated by the steam turbine drives an electrical generator 26 that provides electrical power to the power grid 20. Compressed air from the compressor 28 of the gas turbine is supplied to the gasifier 10. Similarly, steam from the heat recovery steam generator 22 is also provided to the gasifier. Thus, the combined cycle internally generates the steam, compressed air or oxygen and power needed to drive the gasifier 10.
The gasification system may be used in conjunction with fuels such as coal, petroleum coke, residual oil, oil emulsions, tarsands and other similar fuels. This gasification process generates large flows of excess nitrogen association with the production of the oxygen feed for the gasification reaction. A difficulty has been how to best use the excess nitrogen generated from the gasification process. One approach is to return the excess nitrogen to the gas turbine by the injection of nitrogen directly into the combustion section of the gas turbine. Direct injection of nitrogen into the combustion section reduces dramatically the total nitrous oxide emissions from the entire ICGG system.
A difficulty with nitrogen injection is that the excess nitrogen gas generated by the gasifier 10 must be highly compressed to be injected into the combustor of a gas turbine. The nitrogen gas must be compressed to pressure levels at least as great as the levels in the combustion section 14, which is downstream of the compressor 28 of the gas turbine. Substantial power is required to drive the compressor 30 in order to achieve the high levels of nitrogen compression necessary to inject nitrogen into the combustion. The power requirements of the compressor 30 are a relatively high proportion of the total power requirement of the IGCC system.
Compressed nitrogen from the compressor 30 is distributed to the combustion cans of the combustor 14 by a nitrogen injection manifold coupled to the combustor. This nitrogen injection manifold injects nitrogen (N2) into each combustion can of the combustor. The prior art manifolds included nitrogen regulation valves which modulated the flow of nitrogen to the gas turbine combustion cans. The control system 32 for the gas turbine operates the nitrogen flow valves at the manifold to regulate the flow of nitrogen to the combustion section. By adjusting the nitrogen flow valves, the controller 32 regulates the pressure of the nitrogen downstream of the valve flow to the combustor.
The pressure of the nitrogen gas upstream of the flow regulation valve is the supply pressure provided to the flow valves by the nitrogen compressor 30. This supply pressure is relatively constant and must be substantially higher than the pressure of the nitrogen gas supplied to the combustor. The compressor 30 must supply nitrogen gas at a pressure sufficiently high to overcome the pressure loss through the valves and to continually provide the maximum possible nitrogen pressure that would be required for any operating condition of the combustor 14. An additional 10-20 psid (pounds per square inch) above the pressure supplied to the combustor was required to achieve accurate control of the nitrogen flow through the valve. Accordingly, prior nitrogen compressors were driven to provide a continual maximum pressure for the nitrogen gas that is substantially greater than the pressure of the nitrogen when injected into the combustor.
The power required to operate a nitrogen compressor in an IGCC system has been reduced by controlling the pressure of the nitrogen injected to a combustor of a gas turbine at the nitrogen compressor 30. This technique replaces the prior technique of controlling the pressure of the nitrogen being injected into the combustor using nitrogen injection valves at the nitrogen injection manifold of the combustor. The controller 32 for the gas turbine regulates the operation of the compressor 30, such as by adjusting the compressors inlet guide vanes (IGVs), to provide the desired nitrogen gas pressure at the output of the compressor 30. The compressor is relieved of having to overcome the pressure loss through the nitrogen injection valves (which valves are run at or near full open throughout the operation of the nitrogen injection process), and to provide nitrogen at a pressure substantially greater than the nitrogen pressure injected into the combustor.
By using the nitrogen injection compressor to modulate the pressure of nitrogen injected into the combustor, the pressure of the nitrogen output from the compressor can be reduced (as compared to the compressor output required for prior art systems which had to provide nitrogen at a pressure sufficiently above that supplied to the combustor to compensate for the pressure loss through the nitrogen gas control valves and the additional higher pressure required to provide accurate control of nitrogen flow through the valve). This reduction in pressure output by the nitrogen compressor allows for substantially lower power consumption by the nitrogen compressor. This lower power consumption substantially reduces the power costs for IGCC systems.